Magnetic Sensor Using a Microfabricated Soft Magnet Resonator With Active Q-Factor Enhancement

This article presents the design, fabrication, and evaluation of a resonant magnetic sensor based on magnetic torque using a soft magnetic material. The sensor comprises a microfabricated cantilevered Si resonator with a soft magnet and a Cu coil for an excitation magnetic field. A piezoresistive strain sensing element is integrated into the resonator to detect its vibration. Two distinct operation systems of the sensor are proposed, depending on the role of the coil. The first one is a forced vibration method, which uses a magnetic torque driven by the excitation magnetic field using the Cu coil and uses the change in resonant amplitude due to an external magnetic field. The second one uses a quality factor (Q-factor) enhancement based on active damping control, and the coil is used to generate a feedback magnetic field. With this configuration, the feedback magnetic field reduces the effective damping of the resonator. With the forced vibration method, the sensor has a sensing range of ±40 mT and a maximum sensitivity of 6.96 mV/ $\mu $ T with a resolution of 2.87 nT/Hz0.5. Furthermore, when using the Q-factor enhancement method, the sensitivity was improved to 30 mV/ $\mu $ T with a resolution of 1.67 nT/Hz0.5.